Apparatus for fabricating continuous lengths of superconductor

ABSTRACT

A process and apparatus for manufacturing a superconductor. The process is accomplished by depositing a superconductor precursor powder on a continuous length of a first substrate ribbon, overlaying a continuous length of a second substrate ribbon on said first substrate ribbon, and applying sufficient pressure to form a bound layered superconductor comprising a layer of said superconducting precursor powder between said first substrate ribbon and said second substrates ribbon. The layered superconductor is then heat treated to establish the superconducting phase of said superconductor precursor powder.

This application is a divisional of U.S. Ser. No. 08/684,922 now U.S.Pat. No. 5,830,828 filed on Jul. 22, 1996 which is a continuation of theparent application U.S. Ser. No. 08/304,020 filed on Sep. 9, 1994 (nowabandoned).

This invention was made with Government support under DE-AC05-840R21400awarded by the U.S. Department of Energy to Martin Marietta EnergySystems, Inc., and the Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to the fabrication of superconductors. Moreparticularly, the invention relates to the fabrication of continuouslengths of superconductors having at least one superconducting oxidelayer between metallic substrates.

BACKGROUND OF THE INVENTION

The quality of the interface layer between a superconducting materialand a metallic substrate is very important in superconductingoxide/metal conductors. The densest and most highly texturedsuperconducting oxide is generally at the interface between thesuperconducting oxide and the metallic substrate. Consequently, theinterface must be smooth, and in tape geometries, planar.

Preparation of long lengths of superconductor is especially difficult.The powder-in-tube method has been previously utilized with somesuccess. Additionally, recent processes include the utilization ofmultilayered foils coated with superconductor precursor powders that aresealed in a box and unidirectionally rolled. These, and other, methodsfor preparing long lengths of a super conductor result in interfacesbetween the superconducting material and the substrate that are lessthan ideal.

Specifically, fabrication processes such as the powder-in-tube methodproduce long conductor lengths by starting with large billets containingoxide cores several millimeters or more in diameter. Dimensionaluniformity of the core and a smooth interface are difficult to maintainduring the large reduction in cross section necessary for fabrication oflong thin ribbon conductors.

U.S. Pat. No. 5,034,272 to Matsuno et al. and U.S. Pat. No. 5,002,928 toFukio et al. describe methods for depositing oxide substances onsubstrates to form superconductors. Both methods include the depositionof atomized superconductive oxide substances onto a substrate.

Additionally, Shiga et al. (U.S. Pat. No. 5,104,849) describes themanufacture of an oxide superconductor wire. The wire is manufactured byapplying oxide powder to cylindrical stabilizing metal materials havingdiffering diameters. The coated metal materials are then concentricallyarranged. The wire is rolled, before heating the same to produce theoxide superconducting wire. Other superconductors and methods formanufacturing superconductors are disclosed in U.S. Pat. No. 5,208,215to Chen et al., U.S. Pat. No. 5,187,149 to Jin et al., U.S. Pat. No.5,164,360 to Woolf et al., U.S. Pat. No. 5,151,406 to Sawanda et al.,and U.S. Pat. No. 5,059,582 to Chung.

The prior art discussed above neither discloses nor suggests the presentprocess or apparatus for manufacturing continuous strips ofsuperconducting materials having a smooth interface between thesuperconducting material and the substrate. Additionally, the prior artdoes not disclose or suggest a superconductor manufactured by thepresent process. The present process reduces the deformation normallyresulting from the rolling and drawing utilized by the prior artmethods, thus yielding improved texture and less roughness at theinterface between the substrate and the superconducting film.

SUMMARY OF THE INVENTION

An object of the invention is to provide long lengths of superconductorand a process for preparing the same.

Another object of the invention is to provide a method for thepreparation of long lengths of superconductor having a superiorinterface between the superconducting material and the metal substrate.

A further object of the invention is to provide a process and apparatusfor manufacturing long lengths of superconductor having very gooddimensional stability.

These and other objects of the present invention are achieved by thepresent process for manufacturing a superconductor. The process isaccomplished by depositing a superconductor precursor powder on acontinuous length of a first substrate ribbon, overlaying a continuouslength of a second substrate ribbon on said first substrate ribbon, andapplying sufficient pressure to form a bound layered superconductorcomprising a layer of said superconducting precursor powder between saidfirst substrate ribbon and said second substrates ribbon. The layeredsuperconductor is then heat treated to establish the superconductingphase of said superconductor precursor powder.

The limited fabrication required by the present invention results inimproved homogeneity. Specifically, the present invention only reliesupon a two fold reduction in the thickness of the superconductor duringthe fabrication thereof. The homogeniety produced by the presentinvention results in a superconductor having better electricalcharacteristics than those demonstrated by prior art superconductors.Additionally, superconductors made in accordance with the presentinvention demonstrate an absence of non-superconducting materials at theinterface and improved dimensional stability.

Other objects, advantages and salient features of the invention willbecome apparent from the following detailed description which, taken inconjunction with the annexed drawings, discloses the preferredembodiment of the subject invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for fabricating continuouslengths of superconductor. The superconductors made in accordance withthe subject invention are preferably composed of one or more thin,high-temperature superconducting layers between metallic substrates.

With reference to FIG. 1, an apparatus 1 for the continuous fabricationof long lengths of superconductor is schematically illustrated. A firstmetallic substrate ribbon 10 is fed through apparatus 1, while a secondseparate metallic substrate ribbon 12 is simultaneously fed through theapparatus 1 at the same speed as the first ribbon 10. The substrateribbons are long continuous strands. The ribbons are preferably silveror silver alloys (e.g., silver 90-95%/palladium 5-10%), although othermaterials may be used within the spirit of the invention. The ribbonsmay be supplied by any conventional mechanism, for example, conveyers14, used for continuously feeding long strands of materials.

The first ribbon 10 and the second ribbon 12 are simultaneously fed pasta structure 16 for depositing a superconductor precursor powder 18,preferably the bismuth-based, thallium-based, or yttrium-based familiesof high temperature oxide superconductor precursor powders, onto therespective ribbons. The precursor powder 18 may be deposited onto therespective ribbons by spray drying or it may be deposited by directbrush application of a powder slurry in a volatile liquid such asbutanol. In the preferred embodiment, a suspension of the aerosolprecursor powder 18 is stored within a pump assembly 20. Conventionalmechanisms are used to force the precursor powder 18 from the pumpassembly 20.

The superconductor precursor powder 18 is forced through the pumpassembly 20 to a pair of spray nozzles 22, 24. Preferably, the spraynozzles 22, 24 operate at 120 kHz to produce 18 um diameter droplets ofthe superconductor precursor powder 18. The droplets are deposited onthe first and second metallic substrate ribbons 10, 12 as they passbelow the respective spray nozzles 22, 24.

The superconductor precursor powder 18 is applied to the substrateribbons 10, 12 to permit the formation of a continuous coating of thesuperconducting material thereon after fabrication of the superconductoris completed. Preferably, the resulting superconductor should have atleast a 10 micron layer of the superconducting material afterfabrication is completed.

The use of suspensions of aerosol superconductor precursor powder 18 hasat least two advantages. First, the high homogeneity and small particlesize of aerosol powders permits the preparation of thin coatings whichare dimensionally and compositionally uniform. Additionally, highreactivity aerosol powders lead to shorter heat treatment times whichpermit continuous rather than batch fabrication processing.

After the precursor powder 18 is deposited on the first and secondribbons 10, 12, the ribbons move continuously into a low temperaturefurnace 26 where the precursor powder 18 is dried and surfacecontaminants are removed from the precursor powder 18. During this stepthe precursor powder will bond to respective substrate ribbons.

Once the superconductor precursor powder 18 has been appropriatelydeposited on the respective first and second metallic substrate ribbons10, 12, and the ribbons have been appropriately heated, the first ribbon10 and the second ribbon 12 are overlaid to form a mechanical boundlayered superconductor 28 composed of a superconducting layer encasedwithin the first and second metallic substrate ribbons. Formation of thelayered superconductor 28 is achieved by rolling or pressing the layerswith sufficient force to create a bound superconducting layer 28. Therolling or pressing is done by conventional structures 30. Preferably,the edges of the first substrate ribbon 10 and the second substrateribbon 12 are left bare during the deposition step to facilitate theformation of the layered superconductor 28. As a result, the layers ofthe layered superconductor 28 are bound together by both substrate tosubstrate bonding and powder to powder bonding.

If desirable, the edges of the layered superconductor 28 can be folded,or otherwise dressed, to provide a good mechanical bond and to preventfree passage of air borne contaminants.

Finally, the layered superconductor 28 is heat treated to create thedesired superconductor. Specifically, the heat treatment converts thesuperconductor precursor powder to its superconducting phase. Thelayered superconductor 28 is heat treated by passing the same through anappropriate furnace 32. After the layered superconductor 28 is heattreated, the process is completed by rolling 34 the layeredsuperconductor 28 to form a highly textured superconducting core.

The preferred embodiment discussed above, permits the continuousfabrication of long lengths superconductor. That is, feeding,depositing, heating, rolling/pressing, heating, and rolling occurwithout the need to cut the continuous strands of the first and secondribbons 10, 12, until the process is completed. Each of these variablesis also considered when determining the processing rate for thesuperconductor.

If, however, the heat treatment step requires too much time to makecontinuous fabrication of the superconductor feasible, the layeredsuperconductor can be formed in pieces and wound about mandrels. Thewound layered superconductor are then heat treated by convention methodsto convert the superconductor precursor powder to its superconductingphases. By way of this method 1 kilometer lengths of the superconductorcan be manufactured.

In alternate embodiments, the second ribbon can be bare. In such anembodiment, the bare second ribbon is combined with the first coatedribbon in the manner discussed above to form a substrate—superconductingpowder—substrate layered superconductor. Whether the second ribbon iscoated or bare, the edges of the superconductor can be folded orotherwise dressed to provide a good mechanical bond and to prevent freepassage of air borne contaminants. Additionally, multi-layer geometriescan be fabricated by co-rolling several coated ribbons andsuperconductors having a single substrate ribbon are possible.

The process disclosed above results in a smooth interface between thesuperconducting material and the metal substrate. Metallographicexaminations were made of polished cross sections comparing asuperconducting oxide/metal superconductor made in accordance with thesubject invention and a superconducting oxide/metal superconductor madein accordance with a powder-in-tube method. The superconductor made inaccordance with the present invention had a smooth interface between thesuperconducting material and the substrate when compared to thesuperconductor made in accordance with the powder-in-tube method. Thisresults in a superconductor having exceptional electricalcharacteristics, an absence of non-superconducting materials at theinterface, and better dimensional stability.

Having described the preferred embodiment of the present invention, itwill appear to those of ordinary skill in the art that variousmodifications may be made to the disclosed embodiment, and that suchmodifications are intended to be within the scope of the presentinvention.

What is claimed is:
 1. An apparatus for manufacturing a superconductorcomprising: means for depositing a superconductor precursor powder on acontinuous length of a first substrate ribbon; means for overlaying acontinuous length of a second substrate ribbon on said first substrateribbon; means for applying sufficient pressure to form a bound layeredsuperconductor comprising a layer of said superconducting precursorpowder between said first substrate ribbon and said second substratesribbon; and means for heat treating said layered superconductor toestablish the superconducting phase of said superconductor precursorpowder.
 2. The apparatus according to claim 1, further comprising meansfor depositing a superconductor precursor powder on said secondsubstrate ribbon before the formation of said layered superconductor. 3.The apparatus according to claim 1, further comprising means for heatingsaid coated first substrate ribbon prior to forming said layeredsuperconductor.